The purpose of any furnace is to provide an environment at a certain temperature or temperature range for the processing of material. It is important for the proper functioning of a furnace that the temperature be maintained at the desired level. A common means to accomplish this is to determine the furnace temperature with a thermocouple connected to a furnace temperature controller which can increase or decrease the heat input to the furnace. If the thermocouple reads a furnace temperature less than the operating or desired temperature, the furnace temperature controller increases the heat input, and conversely if the thermocouple reads a furnace temperature greater than the operating temperature, the furnace temperature controller decreases the heat input.
Because the thermocouple is physically within the furnace and subjected to high heat, it is very susceptible to failure. Failure, such as by shorting out of the thermocouple, may cause the thermocouple to falsely give an ambient temperature or other low temperature reading even though the actual furnace temperature is within the operating range. In this situation the furnace temperature controller will continually increase the heat input to the furnace because of the false indication that the furnace is below the operating temperature. If left unchecked the temperature of the furnace will continually increase until the temperature exceeds the design limit of the furnace and catastrophic results occur.
In order to avoid such a potentially catastrophic situation, furnace operators generally provide a second safety thermocouple connected to a high temperature controller which is capable of completely shutting down the furnace if a dangerously high temperature is reached within the furnace. Thus, in the event the first furnace temperature control thermocouple fails and gives a false low temperature reading, the furnace temperature will rise only to the preset high cutoff temperature at which point the safety circuit will shut off the heating system.
The above-described conventional system is effective but has certain disadvantages. One disadvantage is that the furnace temperature must increase all the way to the high cutoff temperature before the failure of the temperature control thermocouple can be detected. Another disadvantage is that there is no effective way to check the integrity of the second safety thermocouple except for frequent manual checking of this thermocouple. Thus the situation may arise that the safety thermocouple is not operating correctly but the condition goes undetected because the control thermocouple system keeps the furnace temperature within the proper operating range. However, should the control thermocouple now fail, the furnace temperature would increase and the inoperable safety thermocouple could not shut down the furnace.
It is thus desirable to have a furnace temperature safety system which can overcome the disadvantages of the conventional system.
Accordingly, it is an object of this invention to provide a furnace temperature safety system which can detect a failed control thermocouple without the need for the furnace temperature to increase to a high enough temperature to trip the safety thermocouple system.
It is another object of this invention to provide a furnace temperature safety system which can detect a failed safety thermocouple without need for frequent manual safety checks of the thermocouple integrity.